Functionally Important Structurally-Specific Homodimerization of the Glucagon Like Peptide 1 Receptor

Abstract

The glucagon-like peptide-1 (GLP-1) receptor is an important target for agonist drugs that may be useful in the treatment of type 2 diabetes mellitus. This receptor is a member of class B GPCRs, a group believed to associate with themselves and with each other to form oligomeric complexes. However, the way such complexes might affect the action of these drugs is not known. In the current work, we have studied the ability of GLP-1 receptors to oligomerize and have explored the influence of receptor oligomerization on the effects of both peptide and small molecule agonists that activate this receptor. Bioluminescence resonance energy transfer and bimolecular complementation were used to demonstrate that GLP-1 receptors constitutively form homodimers that were unaffected by occupation with any of these agonists. The lipid-exposed face of transmembrane segment 4 (TM4) was the critical determinant for complex formation, based on observations that competition with TM4 peptide could disrupt such receptor complexes and that TM4 mutants could interfere with the formation of these complexes. The affinities for binding and the potencies for agonist stimulation were lower for the monomeric state than for the dimeric state of this receptor. Treatment with GppNHp shifted the affinity of the dimeric receptor state of the receptor, but not its monomeric state. Negative cooperativity of natural ligand binding was observed only for the dimeric receptor state as well. We also characterized the influence of the dimeric state on signaling activities of various orthosteric and allosteric GLP-1 receptor agonists. The work provides novel insight into the importance of receptor oligomerization on the function of Family B GPCRs.